Peterbilt 378 Back Up Light Bulb Size

Additional Notes

• The Peterbilt 378 uses the 1156 bulb for its back up light across all listed model years, spanning 1988 to 2007.
• The 1156 is a single-filament bulb rated at 12 volts and 21 watts, built on a BA15s bayonet base, and classified under ANSI standards applicable in both the United States and Canada.
• A BA15s bayonet base features a single contact pin, which distinguishes it from the BA15d base used on dual-filament bulbs. The two base types are not interchangeable.
• Because the 1156 is a single-filament design, it serves functions that require only one lighting mode, such as reverse illumination, rather than combined stop and tail functions.
• For the 1997 Peterbilt 378, two separate back up light bulb entries are listed: one general fitment and one specific to vehicles equipped with Halogen Capsule Headlamps, both specifying the 1156.
• The same dual-entry pattern applies to the 1993, 1991, 1989, and 1988 model years, where headlamp configuration variants are listed alongside a general fitment, all calling for the 1156.
• The 1989 model year includes a variant noted as "With 4 Rectangular Headlamps," which represents a distinct headlamp configuration from the Halogen Capsule type listed in other years.
• The presence of headlamp configuration notes alongside back up light specifications indicates that the Peterbilt 378 was offered in multiple trim or equipment variants within a single model year during certain production periods.
• The 21-watt rating of the 1156 places it in a moderate output range for incandescent automotive bulbs, consistent with standard reverse light applications on commercial vehicles.
• The consistent use of the 1156 across all listed configurations confirms that headlamp type does not affect the back up light bulb fitment on the Peterbilt 378.